Fluorescence observation is used in many fields of engineering, biology and medicine in order to visibly discriminate between different structures of an object. Herein, an illumination light filter is disposed in a beam path between an illumination light source and an object to be observed, wherein the illumination light filter allows substantially only light to traverse the filter which can excite a fluorescence of a fluorescent dye. An observation light filter is disposed in a beam path of imaging optics, wherein the observation light filter allows only fluorescent light to traverse the filter whereas light which can traverse the illumination light is substantially not allowed to traverse the observation light filter. Fluorescent structures of the object can be perceived as bright regions in an image which is observed with the eye by viewing into the observation optics or which is recorded by a camera via the observation optics, and non-fluorescing structures of the object remain dark such that structures contained in the non-fluorescing regions cannot be perceived.
It is desirable that also non-fluorescent regions of the object can be seen in the image in order to be able to better determine a position of fluorescent structures relative to the non-fluorescent structures. In this respect, U.S. Pat. No. 6,212,425 B1 suggests to adjust transmission characteristics of the illumination light filter and of the observation light filter relative to each other such that both the fluorescent structures of the object emitting the fluorescent light and non-fluorescent structures reflecting light are visible downstream of the observation light filter.
It has been found that non-fluorescent regions of an object are not satisfactorily visible with such adjustment of the illumination light filter and the observation light filter.